MERCURY ACCUMULATION IN THE BURROWING MAYFLY HEXAGENIA-RIGIDA (EPHEMEROPTERA) EXPOSED TO CH3HGCL OR HGCL2 IN WATER AND SEDIMENT

The accumulation of Hg and its distribution within nymphs of the burrowing mayfly Hexagenia rigida (Ephemeroptera) were investigated experimentally by exposure of nymphs to radiolabelled Hg in laboratory microcosms containing water and sediment. Mercury was introduced into the experimental units either in the overlying water (twice-daily additions) or in the sediment (single addition prior to introduction of the nymphs). At the whole animal level, Hg accumulation varied according to the chemical form of the Hg added and the contamination source. When added in an organic form (CH3HgCl), Hg was accumulated to a greater extent than when added in an inorganic form (HgCl2); the ratio of accumulation between the two compounds exceeded 60 (organic/inorganic exposure) when Hg was introduced via the sediment but was only about 2 when Hg was added to the overlying water. The distribution of Hg among the various body parts of the nymphs indicated that the Hg burdens in two major target organs-the gills and the gut-depended strongly on the initial contamination source. When Hg was added via the sediment, the contribution of the gut to the total body burden (43% for inorganic Hg, 18% for methyl-Hg) was much greater than that of the gills (3 and 5%, respectively). In contrast, when Hg was added to the overlying water, the contribution of the gills to the total body burden (49% for inorganic Hg, 20% for methyl-Hg) exceeded that of the gut (8 and 17%, for the two forms, respectively).